Add "dump" command as a custom "process plugin" subcommand when ProcessMinidump is used.

Each process plug-in can create its own custom commands. I figured it would be nice to be able to dump things from the minidump file from the lldb command line, so I added the start of the some custom commands.

Currently you can dump:

minidump stream directory
all linux specifc streams, most of which are strings
each linux stream individually if desired, or all with --linux
The idea is we can expand the command set to dump more things, search for data in the core file, and much more. This patch gets us started.

Differential Revision: https://reviews.llvm.org/D55727

llvm-svn: 349429